1. Field of the Invention
The present invention relates to laser diodes, and more particularly to optical focusing systems for a laser diode output.
2. Background Technology
High power laser sources are used in a wide variety of applications, including pumping solid state lasers and fiber amplifiers, processing materials, providing optical energy for medical application, etc. In order to achieve a sufficiently high optical output, a high power semiconductor laser source can be implemented in a Master Oscillator Power Amplifier (MOPA) structure. A typical MOPA structure includes a diode feedback (DFB) laser as primary source, followed by a planar waveguide amplifier, both formed on the same semiconductor structure.
The problem with the MOPA device is that its optical output beam is not only divergent (by different amounts in the vertical and horizontal planes of beam propagation), but it also has a very high degree of astigmatism. FIGS. 1A and 1B illustrate a conventional MOPA structure, with a laser diode portion 1 producing an optical output beam 2 that traverses through a planar amplifier portion 3 before exiting the MOPA device. In the horizontal plane of beam propagation (i.e., the plane of the amplifier portion 3), the optical beam 2 is allowed to spread laterally as it traverses through the amplifier portion 3 (see FIG. 1A). However, in the vertical plane of beam propagation, the optical beam 2 is guided through the amplifier portion 3, and does not vertically spread until it exits the MOPA device (see FIG. 1B). Thus, the optical beam 2 leaving the MOPA device has two different effective focal points of origin: one near the exit face of the MOPA (in the vertical plane), and one near the junction of the laser diode portion 1 and the amplifier portion 3 (in the horizontal plane). The difference between these focal points of origin is essentially the length of the amplifier portion 3. This mismatch in focal points of origin of the beam in the two orthogonal planes of beam propagation is commonly referred to as astigmatism.
For most applications, the output of the MOPA needs to be focused to a small spot size (i.e., small beam diameter) with very little if any astigmatism, and with a uniform numerical aperture (measure of divergence and convergence), in both orthogonal planes of beam propagation. For example, it is quite common to focus the MOPA optical beam into a single mode optical fiber having a symmetric transmission mode. It is well known that any given optical fiber has an acceptance numerical aperture associated therewith that if exceeded by an incident light beam would prevent all the light from being properly coupled into the fiber. Focusing optics for these applications must therefore compensate not only for the MOPA output divergence, but also for its astigmatism, in focusing the beam to a small symmetric coherent spot with the proper numerical aperture that matches that of the single mode fiber.
In order to generate a greater amount of amplification, newer MOPA designs include longer amplifier portions, which produce greater astigmatism in the MOPA optical beam. As the astigmatism of MOPA devices increases, it becomes more difficult to design and implement focusing systems that correct for such astigmatism and still provide the desired numerical aperture and spot size for the user's application. Conventional optical focusing systems used to correct large amounts of astigmatism utilize complicated arrangements of optical elements that are difficult to align, sensitive to misalignment, and costly to implement.
There is a need for a simple, cost effective optical solution for correcting relatively large astigmatism in an optical source output beam that is easy to align and produces a symmetric spot size output having the desired numerical aperture with diffraction limited performance.